It's more than harassment , it's a direct threat and I hope they bannish that customer forever. As someone who works in retail P/T , I would take this very seriously . That really sucks , OP !
probably the customer. in my experience 9/10 they are wrong. I used to be a server and wrote down every order and repeated it back. they got their food telling me it was wrong. I showed them where their order was and they still argued with me. Fucking idiots!